The invention relates to a spray nozzle for spray gun for forming a polyurethane layer on a surface by spraying the reaction components for obtaining this polyurethane, comprising a spray piece with a substantially funnel-shaped cavity which, on the one hand, is connected with its widest side to a supply channel of said reaction components and, on the other hand, flows out at its narrowest side in a substantially cylindrical channel having a length between 0.1 and 2 mm, which connects this cavity with a spray opening, a core being detachably placed in the spray piece which allows to lead the reaction components and/or the already formed polyurethane substantially according to a screw or whirling movement through said spray opening.
The invention specially relates to a spray nozzle for spraying, according to the technique of the so-called "airless two-component system without solvent", an elastomer layer of polyurethane, which is preferably light stable and serves especially as an aesthetic covering, such as the lining of a car.
Due to the relatively high viscosity of such a reaction mixture, the relatively considerable thickness of the polyurethane layer, which can be applied advantageously on said surface by one single spray gun passage, and finally the necessary quick gelation of the polyurethane for avoiding the run off of the reacting mixture on the mould surface under the influence of the gravity force, it has been determined that the construction of the spray nozzle of the used spray gun has an important influence on the nature of the obtained polyurethane layer. The spray nozzle can then also be responsible for the fact that micro-air bubbles remain enclosed in the gelified polyurethane layer and, moreover, due to a heterogenity in the spraying of the mixture on the surface, irregularities as for density and other physical characteristics, such as the tone of the so formed layer, can arise.
Hence the invention aims mainly to present a spray nozzle for spray gun having a relatively simple construction and which is consequently easy to reproduce and allows to obtain a constant spray pattern for a well determined reaction mixture, whereby it becomes possible to avoid above mentioned problems on an industrial scale, and thus to obtain a perfect, gelified polyurethane layer on a surface, especially of a mould, by spraying a liquid reaction mixture which comprises polyol and isocyanate and which has preferably a viscosity between 20 and 2000 centipoises.
To this end a funnel-shaped diffuser is connected, in the spray nozzle according to the invention, to a spray opening, which allows to form a divergent liquid jet.
Advantageously, the diffuser has a top angle comprised between 40.degree. and 160.degree., preferably between 80.degree. and 120.degree. and with particular preference of about 90.degree..
In another embodiment of the invention, the core is mounted at the entry of a funnel-shaped cavity, having a tapered part directed towards this cavity, grooves spirally oriented with respect to the axis of said tapered part being provided in the conical wall of this part, which grooves connect the funnel-shaped cavity to the supply channel via cylindrically shaped borings which end, on the one hand, in these grooves and, on the other hand, substantially centrally in the side of the core turned away from the cavity.
In a more specific embodiment of the invention, the grooves are equally distributed over said conical wall of the concerned core part, their number varies from two to ten and the inclination angle of these grooves, with respect to the axis of the core, varies from 15.degree. to 60.degree. and increases as a function of the number of grooves.